ReviewRegulation of the PTEN phosphatase
Introduction
Phosphatase and tensin homologue deleted on chromosome 10 (PTEN) was cloned as a tumor suppressor for gliomas (Li and Sun, 1997, Li et al., 1997, Steck et al., 1997) but is now known to be deleted or inactivated in many other tumor types (Simpson and Parsons, 2001). In addition, PTEN germline mutations are associated with Cowden disease, Bannayan–Zonana syndrome and Lhermitte Duclos disease in which disorganized hamartomas appear in multiple organs (Pilarski and Eng, 2004). Some of the patients with these disorders also show macrocephaly, mental retardation, ataxia and seizures. Recently, a subset of autistic patients with macrocephaly were found to bear PTEN mutations (Butler et al., 2005). Phosphoinositide 3-kinase (PI3K)/PTEN signaling also regulates myocardial contractility and hypertrophy. Therefore, it is not surprising that PI3K/PTEN signaling plays a role in several heart-related diseases, including cardiac hypertrophy, heart failure, preconditioning and hypertrophy (Oudit et al., 2004). Because of its clinical importance, PTEN is now a subject of intense study in many laboratories.
PTEN is composed of two major structural domains, the phosphatase and C2 domains (Fig. 1) (Lee et al., 1999). C2 domains are lipid binding domains with diverse primary sequences, and the PTEN domain was recognized as a C2 domain only after elucidation of the PTEN crystal structure (Lee et al., 1999). In addition, there is a putative phosphatidylinositol (4,5) P2 (PI(4,5)P2) binding domain at the N-terminus and a PDZ ligand consensus sequence at the C-terminus. Hence, like many phosphatases, PTEN includes both an enzymatic domain and additional domains that direct PTEN to subcellular sites.
PTEN is a phosphatidylinositol phosphate (PIP) phosphatase specific for the 3-position of the inositol ring (Maehama and Dixon, 1998)(Fig. 2). Although PTEN can dephosphorylate PI(3)P, PI(3,4)P2 and PI(3,4,5)P3 in vitro, it is likely that PI(3,4,5)P3 is the most important substrate in vivo. PTEN and PI3K have opposing effects on PI(3,4,5)P3 levels and, consequently, opposing effects on cell proliferation and survival (Furnari et al., 1997, Myers et al., 1998). PI(3,4,5)P3 is thought to mediate these effects by inducing phosphorylation and activation of the Akt kinase (Datta et al., 1999, Stocker et al., 2002, Bayascas et al., 2005). In addition, there are reports that PTEN can dephosphorylate itself (Raftopoulou et al., 2004), focal adhesion kinase (Tamura et al., 1998) and the platelet derived growth factor receptor (Mahimainathan and Choudhury, 2004), but genetic studies are needed to elucidate the biological consequences of PTEN dephosphorylation of protein substrates.
PTEN's amino acid sequence and tertiary structure explain a great deal about PTEN's unique enzymatic properties. Phosphotyrosine-specific protein phosphatases and dual-specificity phosphatases that dephosphorylate both phosphoserine and phosphotyrosine share a characteristic signature sequence that is part of the phosphatase active site (Lee et al., 1999, Mustelin et al., 1999). PTEN contains all of the highly conserved residues (green letters, Fig. 1) required for protein phosphatase action but, in addition, has two unique lys residues (red letters) that might interact with negatively charged PIPs. Detailed analysis of the PTEN crystal structure lends further support for PTEN's role as a lipid phosphatase and reveals additional features (Lee et al., 1999). The PTEN active site pocket is wider and slightly deeper than protein phosphatases, allowing both bulky PI(3,4,5)P3 and smaller phosphoamino acids to be substrates. However, the shape of the active site pocket, electrostatic interactions with positively charged sidechains and hydrogen bonds with polar sidechains make PI(3,4,5)P3 binding more favorable. Hence, PTEN has the molecular machinery needed for protein phosphatase action but has diverged from protein phosphatases to favor dephosphorylation of PI(3,4,5)P3.
PTEN binding to membranes is Ca++-independent, and not surprisingly, the C2 domain lacks the canonical Ca++ ligands (Lee et al., 1999). In this respect, the PTEN C2 domain is similar to the C2 domains of Ca++-independent protein kinase C isoforms (Ron and Kazanietz, 1999). The C2 and phosphatase domains associate across a large interface, and many of the residues involved in interdomain contacts are mutated in human cancers (Lee et al., 1999). This finding implies that the integrity of the interface between the two domains is also important for PTEN phosphatase activity.
In this review, we focus on the mechanisms by which PTEN phosphatase is regulated. PTEN is much more than a simple negative regulator of PI3K signaling. In fact, regulation of PTEN may be just as complex as that for PI3K. We summarize studies on PTEN binding to membranes, posttranslational modifications, subcellular distribution of PTEN and binding of lipids and proteins. At each of these stages, PTEN is regulated, thereby determining both PTEN phosphatase activity and subcellular distribution.
Section snippets
Binding of PTEN to phospholipid membranes
As a first step to understanding PTEN function, there have been several studies of PTEN binding phospholipid membranes. PTEN can interact with to pure phosphatidylcholine vesicles (Lee et al., 1999) but shows preferential binding to negatively charged membranes (Das et al., 2003, McConnachie et al., 2003). Furthermore, high salt concentrations inhibit binding, suggesting that PTEN's interaction with biological membranes has an important electrostatic component (Das et al., 2003).
Phosphorylation of PTEN
PTEN is phosphorylated in a variety of cell types, but the consequences of phosphorylation are still being determined. Almost all phosphorylation of PTEN occurs on serine residues. There is a small fraction (< 1%) of phosphothreonine, but no detectable phosphotyrosine (Vazquez et al., 2000, Birle et al., 2002, Miller et al., 2002). The major sites of phosphorylation are ser 370 and ser 385 (Torres and Pulido, 2001, Miller et al., 2002). Phosphorylation of other sites (tyr 240, tyr 315, thr 319,
Subcellular localization of PTEN
Even though PTEN has multiple domains for membrane association (Fig. 1), PTEN in most mammalian cell types does not show an obvious association with the plasma membrane (Lachyankar et al., 2000). Instead, PTEN usually has a punctate cytoplasmic distribution and a substantial presence in the nucleus (discussed below). This finding raises an interesting question. Is PTEN sequestered to cytoplasmic structures or is cytoplasmic PTEN a monomeric protein in a closed conformation (Vazquez et al., 2001
Nuclear PTEN
Although PTEN is cytoplasmic in some cell types (Li and Sun, 1997, Gu et al., 1998), it more commonly is both cytoplasmic and nuclear (Sano et al., 1999, Lachyankar et al., 2000, Perren et al., 2000, Zhang and Steinberg, 2000, Marshall et al., 2001, Tsao et al., 2003, Shoman et al., 2005). In MCF-7 cells, nuclear PTEN is prominent in the G0–G1 segment of the cell cycle (Ginn-Pease and Eng, 2003). Nuclear PIPs (PI(4,5)P2 and PI(3,4,5)P3) have been detected by immunohistochemistry and chemical
Regulation of PTEN by lipids
Several laboratories have reported that PTEN phosphatase activity is enhanced by PTEN’s lipid product, PI(4,5)P2 (Fig. 2), and the reasons for this enhancement are currently being debated (Campbell et al., 2003, McConnachie et al., 2003). Downes and co-workers (McConnachie et al., 2003) proposed that PTEN binds PI(4,5)P2 and, thereby, is localized in the vicinity of the substrate, PI(3,4,5)P3. We proposed that in addition, PI(4,5)P2 activates the phosphatase domain via a conformational change (
Regulation of PTEN by protein binding partners
For many phosphatases, the activity and subcellular localization are regulated by interactions with docking proteins (Faux and Scott, 1996). A number of PTEN-interacting proteins are known (Table 1), and a recent proteomics study suggests that there are additional interacting proteins (Crockett et al., 2005). In this review, we only summarize the best characterized of these interacting proteins.
Summary
PTEN is regulated by a variety of mechanisms, including phosphorylation, oxidation, lipid ligands and protein binding partners. An important issue that remains to be resolved is whether PTEN undergoes a conformational change upon membrane binding as found for some interfacial enzymes. In addition, it has been difficult to associate PTEN regulation with specific physiological events. In this regard, the recent finding of PI3K/PTEN gradients in migrating cells and dividing cells is particularly
Acknowledgments
This study was funded by NIH Grants NS21716 and AR038910.
References (112)
Nucleophosmin/B23, a nuclear PI(3,4,5)P3 receptor, mediates the antiapoptotic actions of NGF by inhibiting CAD
Mol. Cell
(2005)- et al.
Cooperative phosphorylation of the tumor suppressor phosphatase (PTEN) by casein kinases and glycogen synthase kinase 3?
J. Biol. Chem.
(2005) Structure, function and interfacial allosterism in phospholipase A2: insight from the anion-assisted dimer
Arch. Biochem. Biophys.
(2005)- et al.
How accurately can we image inositol lipids in living cells?
Trends Pharmacol. Sci.
(2000) - et al.
Hypomorphic mutation of PDK1 suppresses tumorigenesis in PTEN(+/−) mice
Curr. Biol.
(2005) - et al.
Allosteric activation of PTEN phosphatase by phosphatidylinositol 4,5-bisphosphate
J. Biol. Chem.
(2003) - et al.
The shape of things to come: an emerging role for protein kinase CK2 in the regulation of cell morphology and the cytoskeleton
Cell. Signal.
(2006) Redox regulation of PTEN and protein tyrosine phosphatases in H(2)O(2) mediated cell signaling
FEBS Lett.
(2004)PIP2 and PIP3: complex roles at the cell surface
Cell
(2000)- et al.
More on target with protein phosphorylation: conferring specificity by location
Trends Biochem. Sci.
(1996)